#145421 Fix glyph padding in STB atlas impl.

[johnoneil]

See the associated issue for description and screenshots.

Fixes issue #145421 as described here.

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Changes reported for pull request #51585 at sha e25d538

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Changes reported for pull request #51585 at sha 0936172

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Changes reported for pull request #51585 at sha 09cc270

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Changes reported for pull request #51585 at sha 94560b6

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